🚀 go-pugleaf

In article <1990Oct23.222654.25781@engin.umich.edu> ssave@caen.engin.umich.edu (Shailendra Anant Save) writes:
>
>
>    Has anyone built a digital tachometer from parts?
>

Yes, I have.  I used an optoisolator hooked across the coil _input_ on
my car.  Since it uses 12V to fire the coil once per spark, you can use
it to directly sense RPM.  In a 4 cylinder, 4 stroke car, the coil fires
once per rev (I think).  If your car has separate coils/plug, hook the
optoisolator to one of them, and multiply by 4.  The rest of the circuit
was simple - 7490 counters, 7447 display drivers, latches, and a 555 to
control when the signal was latched/displayed and the counters cleared.
If you've got modern parts around, you can get display drivers/latches/LED's
in one gizmo.  (Try HP optical catalogs).  Constraints include refresh time
on the displays vs resolution vs max and min rpm readings desired. I'll
leave the rest as an exercise for the reader :-).  Hope this helps, it's
been lots of years since I did this.
-- 
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